In printing technology it is known, on the one hand, to print web-shaped printing materials and, on the other hand, to print separated sheet-shaped printing materials. When printing a web-shaped printing material, the printing material, as a rule, is taken off a roll and moved past one or more printing units of a printing machine, wherein a printing medium such as, for example, ink, is applied to the web-shaped printing material.
In digital printing machines, the write cycle—also referred to as the line cycle—required for imaging control has to be adapted with high accuracy to the movement of the printing material web that is moved past a print head of the printing machine. The line cycle, together with the speed of the printing material web, determines the resolution of a printed image in the advance direction of the printing material. To achieve a given resolution and to keep the resolution constant for an entire print job, an exact adjustment between the line cycle and the speed or movement of the printing material web is therefore essential.
Typically, the speed of the printing material web, the speed being the basis of the write cycle, is determined with the aid of a single transport or guide roller or both that is also referred to as the speedometer shaft, and with an encoder that is coupled therewith. Typically, the encoder is configured as a high-resolution incremental encoder that generates equidistant pulses when the speedometer shaft rotates at a constant speed. Based on these pulses and with the exact knowledge of the diameter of the speedometer shaft, the number of bars of the encoder that identifies the number of pulses with one complete rotation of the encoder, and the thickness of the printing material web, it is possible to determine the speed of the printing material and generate a corresponding write cycle. FIG. 2 is a schematic representation of such a configuration.
By the use of a suitable selection of the number of bars of the encoder, the speedometer shaft diameter and the printing material thickness, it is possible, in principle, to adjust to any desired line resolution. In an arrangement as has been described hereinabove, however, the thickness of the web-shaped printing material, as a rule, is not taken into consideration. Printing materials having different thicknesses, however, result in different transport speeds because the thickness of the printing material contributes to the effective radius of the speedometer shaft. If such differences are not taken into consideration in the aforementioned system, inaccuracies result regarding the adjustment between the line cycle and the speed of the printing material when the printing machine is operated with printing materials having different thicknesses. Ultimately this leads to an elongation or compression of an image at the time of imaging because the write cycle is not exactly adjusted to the actual transport speed of the printing material.
Hereinafter, the influence of the thickness of the printing material on the actual web speed of the printing material shall be explained in greater detail.
For a line resolution R, the following applies:
                    R        =                  E                      Π            ⁡                          (                                                D                  T                                +                T                            )                                                          (        1        )            wherein E is the number of bars of the encoder, DT is the diameter of the speedometer shaft and T is the paper thickness (see FIG. 2).
Depending on the selection of the speedometer shaft diameter, the actual line resolution varies by 3‰ to 5‰ over the thickness range relevant to the roll printing machines for printing materials that can be used. In an A3 sheet having a length of 420 mm this corresponds to an absolute length error of 1.2 mm to 2 mm. An error of this order of magnitude is intolerable in high-quality printed products.
In order to solve this technical problem, high-resolution incremental encoders or encoders are frequently used, whereby their output clock rate is scaled via a divider logic to the desired line cycle. The scaling factor required for a specific paper thickness is typically determined by the machine operator by measuring test images and is manually set via a control panel. However, this procedure is time-consuming and labor-intensive, and the result is dependent on the care with which the machine operator is working. Furthermore, this procedure has to be performed for each new roll of printing material and can thus not respond to changes within a roll of printing material.